It is often desirable to limit the power dissipation in an output transistor in order to protect such transistor from damage. For example, an output transistor of an audio amplifier can be exposed to excessive power dissipation under various combinations of input signal and output load conditions. Also, transistors employed in a voltage regulator can be exposed to excessive power dissipation under various combinations of unregulated input voltage and output load conditions.
It is known in the art to limit the maximum power dissipation in an output transistor by monitoring the instantaneous power and providing degenerative feedback so as to prevent the power dissipation from exceeding a predetermined limit. In particular, output current I.sub.O is sensed by a current sensing resistor inserted in series with the collector-emitter path of the output transistor. The collector-emitter voltage V.sub.CE is sensed and multiplied by I.sub.O to obtain a product representing the instantaneous power dissipation. The product is compared to the predetermined limit. If the limit is exceeded, a feedback signal is applied to reduce the power dissipation. The latter may be accomplished, for example, by diverting signal current drive from the base electrode of the output transistor. A disadvantage of the prior art power limiting approach described above is that the use of a current sensing resistor reduces the available output voltage, and therefore the available output power. Moreover, circuits for obtaining the product of two analog quantities tend to be complex.